1. Field of the Invention
This invention relates to gas turbine combustor liners with dilution holes, and more particularly, to combustor liners which are made from a single piece near net shape form such as a forging or a casting and having asymmetric cylindrical dilution holes machined in the liner.
2. Description of Related Art
Combustor liners are generally used in the combustion section of a gas turbine engine which is located between the compressor and turbine sections of the engine. Combustors generally include an exterior casing and interior combustor radially inner and outer liners between which fuel is burned producing a hot gas usually at an intensely high temperature such as 3,000 degrees F. or even higher. To prevent this intense heat from damaging the casing structure before it exits to a turbine, a combustor liner is provided in the interior of the combustor. The combustor liner prevents the intense combustion heat from damaging the surrounding engine. Various cooling techniques are used to cool the liner such as film cooling.
Gas turbine engines often have a dilution air zone in the combustor immediately downstream of a flame zone. Dilution air is injected into the combustor to control the temperature of the hot gases before they enter the nozzle on their way to the turbine blades. Dilution air is injected into the combustion zone through dilution holes in the liner to control the temperature of hot gases in the combustor. This has conventionally been handled by injecting dilution air into the hot gases well upstream of the outlet of the combustor in order to ensure thorough mixing and cooling prior to entry into the nozzle. In short combustors, this method of controlling the temperature of the hot gases is not satisfactory in every respect. Dilution hole rings and bosses have been used to inject the dilution air into the dilution air zone which is conventionally located directly downstream of the fuel injectors and upstream of the outlet of the combustor. One type of dilution hole ring is illustrated in U.S. Pat. No. 6,212,870 on the inner combustor liner (numeral 18 in FIG. 1 of the patent) and is illustrative of the inner combustor liner used on the GE90 aircraft gas turbine engine.
A cooling air film along the walls of the combustor liner is often used to cool the liner. Techniques for cooling the liner include nuggets formed in the liner and having film cooling air slots open in an axially aftward direction (downstream) to help produce an cooling air film. Multi-hole film cooling holes are also used for film cooling the liner.
One engine produced by the General Electric Company has 60 dilution hole rings which are welded into the GE90 inner combustor liner. The dilution hole rings or bosses have an aerodynamic purpose which is to provide improved altitude ignition performance and they prevent penetration of the dilution air into the inner swirler recirculation zone, which would reduce altitude ignition capability. The dilution hole rings redirect the inner liner primary dilution away from the inner swirler recirculation zone, thus, allowing altitude ignition capability. The present dilution hole rings are fabricated from barstock and TIG welded into holes in the inner liner. Each weld must be inspected using florescent penetration and also x-rayed. Distortion due to the welding requires that each liner be resized to restore the liner to its original contour and dimensions. The complex shapes and machining required to produce these liners negatively effects their cost.
An annular gas turbine engine combustor liner has an axially extending single piece annular shell circumscribed about a first axis of revolution. The shell has a hot side and a cold side, an annular dilution hole section, and a plurality of asymmetrical cylindrical dilution holes extending through the section. An exemplary embodiment of the invention includes pockets circumferentially interdigitated between the dilution holes and extending into the shell from the cold side such that the pockets form dilution hole bosses between the pockets through which the dilution holes extend. Each of the dilution holes is circumscribed about a second axis of revolution which is not parallel to the first axis of revolution. The cold side is a first frusto-conical surface having a first conical angle with respect to the first axis, the hot side is a second frusto-conical surface having a second conical angle with respect to the first axis, and the first and second conical angles have different values. In the exemplary embodiment of the invention, the dilution hole section thickens in an axially aftwardly direction. The liner may further include forward and aft annular cooling nuggets located forward and aft of the dilution hole section and having annular film cooling slots which are open in the axially aftwardly direction. The shell is machined from a single piece integrally formed near net shape form such as a forging or a casting or, in a more particular embodiment, a centrifugal casting.
The invention includes a method for making the annular gas turbine engine combustor liner and the method includes forming a single piece integrally formed near net shape form for the axially extending single piece annular shell circumscribed about the first axis of revolution then machining the annular shell having a hot side and a cold side and the annular dilution hole section of the shell, and then drilling the plurality of asymmetrical cylindrical dilution holes extending through the section. An exemplary embodiment of the invention includes machining the pockets to be circumferentially interdigitated and centered between where the dilution holes are to be drilled before drilling of the dilution holes. The pockets are machined into the shell from the cold side such that the pockets form dilution hole bosses centered between the dilution holes. Machining the annular shell includes machining the dilution hole section to be frusto-conical in shape such that the cold side is a first frusto-conical surface having a first conical angle with respect to the first axis and the hot side is a second frusto-conical surface having a second conical angle with respect to the first axis, and the first and second conical angles have different values. The shell is machined such that the dilution hole section thickens in an axially aftwardly direction. Forward and aft annular cooling nuggets forward and aft of the dilution hole section are machined into the shell including annular film cooling slots which are open in the axially aftwardly direction in the nuggets. The near net shape form is made by forging or casting or, in a more particular embodiment, by centrifugal or spin casting.